The present invention claims priority from Japanese Patent Application No. 11-036446 filed Feb. 15, 1999, the contents of which are incorporated herein by reference.
1. Field of the Invention
The present invention relates to a radio communication system in which a service area is composed of a plurality of cells, a base station is provided in each of the service cells and a plurality of channels are assigned to each base station for performing communication between a plurality of mobile units and these base stations.
2. Description of the Related Art
In a mobile radio communication system, it is possible to increase the number of mobile units (users) by effectively utilizing a limited number of communication channels. Therefore, it is important to utilize the channels effectively. Besides the conventional fixed channel assignment method in which communication channels, which can be used always, are preliminarily assigned to every base station, a method for searching a communication channel usable on demand and using it has been proposed as a method for effectively utilizing communication channels. The latter method is referred to as xe2x80x9cdynamic channel assignment method for adaptively re-utilizing channelsxe2x80x9d.
As a concrete example of the dynamic channel assignment method, Japanese Patent Application Laid-open No. H6-53901 discloses a technique titled xe2x80x9cIdle Channel Detection Method for Mobile Communicationxe2x80x9d in which interference wave levels of all of communication channels assigned to a system are measured in an arbitrary sequence and communication channels whose interference wave levels are lower than a certain constant level are determined as idle channels, which are in idle state and can be utilized.
In the disclosed conventional idle channel detection method for mobile communication system, which is constructed with a service area composed of a plurality of cells, base stations provided in the respective cells, a plurality of mobile units and a plurality of communication channels assigned to the system for performing communication between the mobile units and the base stations, the mobile unit or the base station measures interference wave levels in all of communication channels assigned to the system in an arbitrary sequence and communication channels whose interference wave levels are lower than a certain constant level are determined as idle channels. The communication is performed between the mobile units and the base stations by utilizing the idle channels.
In the conventional method, a plurality of different idle channel search sequence tables of the communication channels having interference wave levels to be measured are prepared and each of the idle channel search sequence table is assigned to base stations separated from each other by a constant distance. The base station or the mobile unit measures the interference wave levels of the communication channels in sequence according to the assigned idle channel search sequence table. In a case where the mobile unit measures the interference wave levels, the base station notifies the mobile unit of the assigned sequence table such that the base station or the mobile unit can use an idle channel, which is initially detected by the mobile unit according to the idle channel search sequence table supplied from the base station, for the communication therebetween.
An operation of the idle channel detection method of the conventional mobile communication system will be described with reference to FIGS. 8 and 9, in which a mobile unit 1 communicates with one of base stations 8A, 8B and 8C, which are connected to a management server (not shown) through a cable network (not shown), while moving from a cell having one base station to another cell having another base station.
In FIG. 8, it is assumed that the mobile unit 1 communicates with the base station 8A provided in a service cell 8a. It should be noted that each of base stations denoted by 8A, 8B and 8C can use the same communication channels repeatedly. FIG. 9 shows an idle channel search sequence table, which is provided in each of the base stations and indicates sequences of the communication channels whose interference wave levels are to be measured to find an idle channel in each base station. As shown in FIG. 9, communication channels 1 to 10, 20, 30, 19, 29, . . . , 11, 21 are searched in the sequence for idle channel in the base station 8A, communication channels 11 to 20, 30, 10, 29, 9, . . . , 21, 1 are searched in the sequence for idle channel in the base station 8B and communication channels 21 to 30, 10, 20, 9, 19, . . . , 1, 11 are searched in the sequence for idle channel in the base station 8C.
A case where an amount of traffic is small and the number of channels used in each base station is smaller than 10 will be considered. For example, if the mobile unit 1 searches an idle channel when the base station 8A is using channels 1 to 5, the base station 8A requests the management server the idle channel search sequence table first. Then, the base station 8A searches the communication channels from channel 1 according to the idle channel search sequence table and repeats the search until the idle channel is found. In such case, the channel 6 may be detected as an idle channel detection.
That is, since the respective base stations detect idle channels according to the idle channel search sequence table, it is probable that channels used in the base station 8A become busy from the initial channel 1, channels used in the base station 8B become busy from the initial channel 11 and channels used in the base station 8C become busy from the initial channel 21. Therefore, if the traffic is small, the possibility that all of the channels 1 to 10 are used in both the base stations 8B and 8C may be low and the possibility that the channel 6 is an idle channel may be high.
If the amount of traffic is large and the channels 1 to 10 are being used in the base station 8A, there is no idle channel in the base station 8A since the limit capacity of the system is 10 channels. However, the possibility that there is an idle channel or channels if there is a base station which is using channels the number of which is smaller than 10. When the mobile station 1 performs the idle channel detection, the mobile unit 1 requests the idle channel search sequence table from the base station 8A, first. Then, the mobile unit 1 measures the idle channel from the channel 1 according to the idle channel search sequence table and repeats the measuring processing until an idle channel is detected.
Since the base station 8B uses channels from the channel 11, the possibility that the channel 20 among the channels 11 to 20 is not used is the largest. Similarly, in the base station 8C, the possibility that the channel 30 is idle is high. Therefore, if the mobile unit 1 performs the measurement in a sequence of channels 20, 30, 19, 29, . . . according to the idle channel search sequence table supplied from the base station, it is possible to detect an idle channel with minimum number of measurements.
However, there are the following problems in the described prior art. That is, since, in order to detect an idle channel in the mobile unit, interference wave levels must be measured (detected) from the initial channel according to the idle channel search sequence table until an idle channel is detected, it takes a long time to detect an idle channel when the amount of traffic is large.
Further, since the idle channel search sequence tables assigned to the respective base stations are fixed, the idle channel detection in the mobile unit must be performed from initial channels of the respective base channels according to the idle channel search sequence tables without taking channels, which are being used, into consideration.
An object of the present invention is to provide a mobile unit radio communication system, in which a scanning of idle channels in base stations located around a mobile unit can be performed efficiently in the mobile unit to thereby improve the search speed.
In the present invention, when a communication link between a mobile unit and a base station is established, the base station itself requests a management server an update of its own idle channel information through a cable network. Upon the request, the management server updates the idle channel information of the base station and transmits an idle channel information of peripheral base stations located peripherally of the base station to the mobile unit through the cable network and the base station, which requested the update of own idle channel information.
The mobile unit searches channel frequencies, which are not used by other base stations, by searching idle channels of the peripheral base stations, selects an idle channel from the idle channel information of the peripheral base stations, which is given by the management server, and uses the idle channel frequency thus selected as the frequency of the mobile unit, without searching idle channel of the peripheral base stations.
Therefore, there is no need for the mobile unit of searching idle channel of the peripheral base stations and it is enough to select an idle channel from the idle channel search sequence table storing the idle channel information of the peripheral base stations. Consequently, it becomes possible to realize a mobile radio communication system having an efficient idle channel detector.